Generally, the present invention is concerned with the treatment of wastewaters which contain biodegradable solids. Such wastewater may emanate from sewage collection systems, oil refineries, coke plants, paper making plants, canneries, food processing plants and the like. The treatment of these organic dissolved and suspended materials is typically accomplished by a process commonly classified as an aerobic treatment process. Removal of the organic material by this process is accomplished by two general mechanisms. First, impurities are adsorbed or absorbed at the interface between the associated biomass and the wastewater. Second, the biomass decomposes these organics through oxidation. The resulting increased biomass or sludge consisting of accumulated microorganisms is generally separated from the organically stabilized liquid. Most of the biomass is generally returned to the process to continue the process and the excess sludges are periodically removed from the system.
In conventional biological treatment systems, the major components are typically an aeration basin and a clarifier tank. The aeration tank may be rectangular or circular and contain means for continually circulating the mixed liquor (suspended solids and waste liquid) within the tank with the addition of oxygen or air to promote micro-organism growth. The aeration basin may also be generally oval in shape and define a trough-like channel having bottom and spaced upstanding side walls for retaining and circulating the mixed liquor in a continuous substantially closed flow path, which is often referred to as an "oxidation ditch". The mixed liquor is continuously circulated by means of rotating o brushes, discs, turbines or the like, at a flow velocity to maintain the solids in suspension. Additional air or oxygen may also be added to the circulating mixed liquor to promote micro-organism growth.
In the oxidation ditch system, a clarifier is required to separate suspended solids from the mixed liquor and to withdraw clarified liquid. The clarifier may be a separate unit located adjacent the oxidation ditch, and serves as a settling tank for separating suspended solids from the mixed liquor by gravity. The clarified liquid may be disposed of or reused, while the settled biomass remains in the clarifier, from which it may be disposed of as waste sludge, or recycled to the oxidation ditch to maintain the proper balance between organic loading and biological microbial mass solids in the mixed liquor. The separate clarifiers typically require pumping means to transmit mixed liquor from the oxidation ditch to the clarifier and/or pumping means to transmit the settled biomass from the clarifier back into the oxidation ditch.
In U.S. Pat. No. 4,614,589 a biological aerobic treatment system is disclosed that includes a streaming specific gravity separator located in the oxidation ditch. The separator separates and removes clear liquid from the mixed liquor flow path in a unique manner. U.S. Pat. No. 4,614,589 is assigned to the same assignee as the present invention.
U.S. Pat. Nos. 4,303,516, 4,383,922 and 4,446,018 also disclose oxidation ditches that include internal clarifier devices.
Oxidation ditches that include brush aerators to circulate the mixed liquor stream impart energy to mix and keep the contents of the mixed liquor stream flowing at the surface and a depth of not more than about fourteen inches. This energy is typically imparted most often in at least two locations in the elongated sections of the flow channel defined by the oxidation ditch adjacent the downstream ends of the end sections of the flow channel. This concentrated energy is extremely turbulent and must be changed into smooth, uniform flow to a depth of five feet to twelve feet. Current efforts to create more uniform velocities have resulted in uneven velocities throughout the flow channel. Most often the result has been a high surface velocity and a zero bottom velocity.
Present efforts to obtain uniform velocity have included the use of flow baffles and turning baffles located in the flow channel. Referring to FIG. 1, a schematic representation of an oxidation ditch 10 is shown that illustrates the prior art efforts to establish a more uniform flow velocity in the flow channel. Oxidation ditch 10 defines a flow channel 12 having a pair of end sections 14 and a pair of elongated sections 16 extending therebetween. A center wall 18 separates the sections 16. A brush aerator 20 extends across each of the elongated sections 16 a short distance downstream of each end section 14. The direction of flow in channel 12 is indicated by arrows. A flow baffle 22 is located across each section 16 downstream of each of the brush aerators 20. The flow baffles 22 are intended as a means to force more surface velocity to a greater depth. A turning baffle 24 is located in each of the end sections 14. Turning baffle 24 is spaced inwardly from the outer side wall defining the channel and extends from the bottom of the channel to an elevation above the surface of the flowing mixed liquor stream. The downstream or outlet edges of the baffles 24 terminate a short distance upstream of the aerators 20. The distance between the inlet and outlet edge of the baffles 24 and the inside and outside walls of the channel may be equal. Alternatively, it has been proposed to make the distance between the baffle and the outside wall at the inlet edge at one-third the width of the channel and the distance between the baffle and the outside wall at the outlet edge at one-half the width of the channel. The later arrangement causes the baffles to collect uneven quantities of the mixed liquor stream at the inlet end and results in directing more liquid to the inside of the brush aerator where the entering velocity is low.
Tests were conducted on an exemplary oxidation ditch of the type shown in FIG. 1 to determine velocity data at various locations in the flow channel downstream of the brush aerator. The locations were at four different depths at inside and outside portions of the channel. In a first test the oxidation ditch included two aerators 20, two turning baffles 24 and one flow baffle 22. In a second test the second flow baffle was included. The test data is shown in Table 1.
TABLE 1 __________________________________________________________________________ 1 FLOW AND 2 TURNING BAFFLES 2 FLOW AND 2 TURNING BAFFLES DEPTH VELOCITY INSIDE VELOCITY OUTSIDE VELOCITY INSIDE VELOCITY OUTSIDE __________________________________________________________________________ 2' Down 1.4'/sec. 1.1'/sec. 1.4'/sec. 1.8'/sec. 4' Down 0.8'/sec. 1.0'/sec. 0.6'/sec. 0.5'/sec. 6' Down 0.6'/sec. 0.6'/sec. 0.3'/sec. 0.2'/sec. 8' Bottom 0.0'/sec. 0.6'/sec. 0.0'/sec. 0.0'/sec. AVERAGE VELOCITY 0.8'/SEC. AVERAGE VELOCITY 0.6'/SEC. __________________________________________________________________________
Table 1 shows velocity downstream of the brush aerator measured at eight different points on a cross-sectional plane in the ditch. Table 1 also shows an average cross-sectional velocity.
As depicted in phantom lines in FIG. 1, the turning baffle 24 causes a vertical vortex at the downstream end thereof under the aerator 20 due to uneven velocity or energy leaving the baffle. The flow baffle 22 tends to create and reinforce the vortex. The flow baffle 22 also impedes the liquid flowing around the ditch in much the same manner as the positioning of a flat plate perpendicular to the flow would impede the flow.
There is a need for a better means to control the velocity of the flowing mixed liquor in an oxidation ditch in a manner that establishes a more uniform and increased cross-sectional velocity throughout the flow channel.